The present invention relates to a surface acoustic wave device using lithium tetraborate single crystal (Li.sub.2 B.sub.4 O.sub.7).
The surface acoustic wave device is a circuit element for processing signals by converting an electric signal to a surface wave, and is used in filters, delay lines, etc. Usually the surface acoustic wave device comprises a metal electrode which is the so-called interdigital transducer (IDT) disposed on a piezoelectric elastic substrate (piezoelectric substrate), whereby an electric signal is converted and reconverted into and from a surface wave. The surface acoustic wave device mainly uses, as the surface acoustic wave, surface acoustic waves called Rayleigh waves which propagate over elastic surfaces without losses.
Materials of the substrate of the surface acoustic wave device are quartz, lithium tantalate (LiTaO.sub.3), etc. Quartz has good temperature stability but poor piezoelectricity. Lithium tantalate has good piezoelectricity but poor temperature stability. Recently lithium tetraborate single crystal has been noted as a material satisfactorily having these properties (e.g., Japanese Patent Publication No. 44169/1990 and Japanese Patent Publication No. 40044/1988).
But the surface acoustic wave device using lithium tetraborate has found it difficult to process higher frequencies of signals.
A width of the finger electrodes of an interdigital transducer (IDT) and a gap between the respective finger electrodes and their adjacent ones are set at 1/4 of a wave length of a surface acoustic wave corresponding to a signal frequency to be normally processed (.lambda./4). But the propagation velocity of Rayleigh waves over lithium tetraborate single crystal is about 3400 m/sec, and for the processing of signal frequencies above 1 GHz, the finger electrode width and electrode gap must be below 1 .mu.m. Accordingly fabrication yields of the interdigital transducer lower, which makes the fabrication of the surface acoustic wave device very difficult.
But the fabrication of the interdigital transducer is facilitated by raising an acoustic velocity (propagation velocity) of the surface acoustic waves to thereby allow the finger electrode width and the gap between the finger electrodes to be increased. It is proposed to use, as the surface acoustic waves, leaky surface acoustic waves, the so-called leaky waves, which propagate, radiating a part of their energy in the direction of depth of an elastic substrate. Generally leaky waves have too large propagation losses to be used in surface acoustic wave devices. But their propagation losses can be reduced depending on specific cut angles or propagation directions, and leaky waves can be used. For example, 36.degree. Y-X lithium tantalate is proposed.